Literary Digest polled 10 million Americans, 2.4 million responded.
N = 128 million, n = 2.4 million
Prediction: 43% for FDR
Result: 62% for FDR
Literary Digest surveyed
These groups have a much higher income on average than the typical american. In 1936 the Great Depression is still in full swing, so the typical (poorer) american was more supportive of FDR.
Their sampling method was biased (not representative).